High power semiconductor lasers and semiconductor optical amplifiers have large heat fluxes, typically 4 kWatt/cm.sup.2 or more, and are implemented in fragile material systems with moderate to poor thermal conductivity. This poses a significant device packaging challenge, because the die attach process must provide excellent heat transfer from the entire active area to the heatsink and must not produce excessive stresses (e.g., more than 20-50 MPa) on the die. The device packaging technology must also not degrade the quality of the emitted beam, which is ideally limited only by diffraction due to the finite size of the emitting aperture.
The thermal resistance of the die attach process is important because the threshold currents of semiconductor laser diodes and the gains available from semiconductor laser diode amplifiers are temperature sensitive. The temperature sensitivity of a semiconductor laser is quantified by the empirical expression: EQU I.sub.threshold (T)=I.sub.threshold (T.sub.ref).multidot.exp ((T-T.sub.ref)/T.sub.o) (1)
where T is the temperature of the active region of the laser, T.sub.ref is a reference temperature, and T.sub.o, is the characteristic temperature of laser threshold (an empirically-determined parameter which is itself dependent upon temperature and device geometry). As seen from equation (1), the most temperature sensitive semiconductor lasers, such as InGaAsP devices operating at 1.55 .mu.m, exhibit low T.sub.o values (40 to 70.degree. C.). Heat transfer from the active region is poor in optoelectronic devices because the thermal conductivity of the active region, substrate, and cladding layers is low. The substrate is typically 90 to 120 .mu.m thick and the epitaxial layers containing the active region are typically 3 to 4 microns thick. It is thus necessary to mount the die with the epitaxial side toward the heatspreader or heatsink in order to minimize the thermal resistance. This so-called epi-side down mounting configuration reduces the yield of the die attach process because it greatly increases the chances of solder bridging the submicron thick, intrinsic active layer at the device sidewalls or facets. For GaAs/Al.sub.x Ga.sub.1-x As devices which are mounted epi-side down, the thermal conductivity of the Al.sub.x Ga.sub.1-x As cladding sets a lower limit on specific thermal resistance for the die attach method. For example, a broad area device with a t=1.5 .mu.m thick cladding layer and a thermal conductivity of .kappa.=11 Watt/(m.degree. C.) (Al.sub.0.5 Ga.sub.0.5 As), and with negligible heat transfer through the substrate, will exhibit a specific thermal resistance of at least: EQU .rho.=t/.kappa.=1.36.multidot.10.sup.-3 .degree. K.multidot.cm.sup.2 /Watt(2)
A packaged high power semiconductor laser or optical amplifier must certainly have a low thermal resistance to reduce the average temperature of the die but must also be free of any significant localized heating or hotspots. Hotspots due to solder voids are especially problematic, because voids with diameters of even a few microns can lead to beam filamentation in high power devices and significantly degrade beam quality. Hotspots may also occur within about 5 .mu.m of the facets of high power GaAs/AlGaAs devices and visible lasers. Localized heating, either at solder voids or at the device facets, places stringent requirements on the die positioning, interfacial flatness, heat spreading properties of the die attach process, and the design of the heatsink in high power devices.
Semiconductor lasers and amplifiers exhibit low fracture strength and are easily damaged by stress. For this reason, soft, low-melting temperature solders are often used to reduce the stress caused by the coefficient of thermal expansion (CTE) mismatch between the die and submount or heatsink. Soft solders, such as indium and indium-based alloys exhibit low yield stresses and are thus widely used for optoelectronic die attach. The slow, continuous deformation of these soft solders, known as creep, reduces the long term reliability of packages which are subjected to temperature cycling since fatigue occurs in the solder layer. Hard eutectic solders, such as 80-20 gold-tin, 88-12 gold germanium solder, and 97-3 gold silicon, show less creep and have higher yield strengths but also have higher melting temperatures and therefore cause significant stress in the die.
The requirements for heatspreading and stress minimization in optoelectronic packages are usually met by incorporating submounts between the semiconductor die and the heatsink. Type IIa natural diamond or high quality CVD diamond submounts may be used as well as silicon, cubic boron nitride or aluminum nitride. The heatspreading benefits of these submounts cannot be realized without a uniform, low thermal impedance die attach method.
The existing technology, however, is not free of voids in the semiconductor-solder interface. The existing technology results in poor process control, poor yield, high thermal impedance, and hotspots.
It is therefore desirable to provide a well-controlled, high yield, void-free die attach method for the epi-down mounting configuration of high power semiconductor lasers and optical amplifiers. Such a method should consistently yield absolute thermal resistances of about 1.5.degree. C./Watt on tapered semiconductor optical amplifiers and specific thermal resistances of 4.multidot.10.sup.-3 .degree. C. cm.sup.2 /Watt for lasers and optical amplifier devices. The method should result in wide process margins, exhibit excellent yield and repeatability, while relaxing the requirements on heatsink flatness and roughness. The method should desirably be well suited for laser die attach to heatspreading diamond submounts or advanced composite materials.